Perturbation, chiral structural

Supramolecular architectures are highly sensitive to chiral perturbations in general, and in systems that form liquid crystals in particular. Small amounts of enantiopure guest molecule added to a nematic host can induce a transition to a cholesteric phase, and the helical organization in the mesoscopic system is very sensitive to the structure of the guest molecule. Chiral amplification was successfully achieved in such liquid crystals, using CPL as the chiral trigger for the phase transition [183]. 

For instance, solvent effects can represent much more than just a small perturbation of the electronic structure of a molecule. Sometimes these effects can be strong enough that the chiroptical response can be dominated by their influence on the chiral solute. For instance, effects that should be considered are how a solvent shell perturbs the electronic structure of the solute (and therefore its chiral response [155]), or the possibility of a solute transferring its chirality to the surrounding solvent shell (influencing one particular solvent molecule [151] or the entire solvent shell [156]). Here and in the following discussions it is assumed that the solvent itself is not chiral, and therefore the only contributions to the chiroptical response of the solution are from the solute or from chirality induced by the solute in the solvent shell. 

In deciding whether analytes are CD-active, it is not always a simple matter to inspect a molecular formula and be certain that the chromophore and the chiral center are mutually located in a manner that produces activity. Even if the molecular structure suggests that a chirally perturbed chromophore is present, the substance might only be available as an achiral racemic mixture and therefore is not detected by CD. 

Two factors contribute to the success of this reaction the outstanding enantio-selectivity achieved, and efficiency of the catalyst (i.e, high turnover). The above analysis emphasizes only the former, but the latter also varies with the nature of the chiral bisphosphine ligand and the structure of the substrate. The structural features of the substrate and the catalyst are mutually optimal in the example cited above. Perturbation of any of these features usually lowers either the enantioselectivity or the turnover rate. The range of substrates that are amenable to asymmetric hydrogenation with this catalyst system is, therefore, limited. Figure 7.9 illustrates the classes of substrate that can be accomodated by cationic rhodium bisphosphine catalysts [104]. For a more extensive summary, see ref. [110]. 

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